1. Field of the Invention
The invention relates to a diffuser to be provided on the discharge side of a turbo pump, in particular, to the structure of the diffuser's vanes, and the method of manufacturing a diffuser with a structure having such vanes.
2. Description of the Related Art
A diffuser is sometimes provided on the discharge side of a turbo pump for efficiently converting the velocity energy generated by the impeller of the pump into pressure energy. A diffuser has a plurality of vanes that form multiple flow paths for reducing the velocity of the subject fluid discharged from the impeller and increase the pressure. The shape of a vane, i.e., the shape of the flow path, is determined based on the design point of the pump. As a result, when the pump is operating under conditions different from said design point, the flow of the fluid in the diffuser flow path is different from the flow under the design point and causes pump operation problems in some cases. When the flow rate reduces, in particular, the flow of the subject fluid peels off from the vane and causes a stall. This stall does not occur simultaneously in all flow paths but rather in some of the flow paths, or often only in one flow path. This distorts the symmetry of the fluid flow relative to the centerline of the pump and generates a radial force against the pump. Moreover, as time passes, the flow path where the stall occurs rotates around the axis of the pump, consequently causing the direction of said radial force acting on the pump to change and generating a vibration of the pump. Such a phenomenon that the area where the stall is occurring rotates around the axis of the pump is called a rotational stall.
A method of suppressing the abovementioned rotational stall is disclosed in Japanese Patent No. 2735730. The patent shows an embodiment where each vane of the diffuser consists of two parts separated by a gap along the direction of the flow of the fluid. As the discharge flow reduces, a circulating flow develops in which the fluid circulates back to the diffuser's inlet through the gap of the vane, thus causing the apparent flow rate to increase, suppressing the stall and the rotational stall.
When a diffuser's vane is divided into two parts separated in the direction of flow, it generates a circulating flow to suppress the stall, but it also causes a drop in the pump efficiency. Therefore, it has been customary to seek an optimum position of the gap and its width (space that separates the two parts of a vane), a compromise that provides a smaller pump efficiency drop and efficient stall suppression, by trial and error. However, once the position and width of a gap is formed by machining, it is difficult to change it later. Although the width can be changed, it means a further drop of the pump efficiency. As the occurrence of the rotational stall varies with the installation condition of the pump, said trial and error method of seeking an optimum position and width makes it necessary to produce multiple diffuser prototypes or may cause to reduce the pump efficiency unnecessarily.
The present invention is intended to solve these problems by conducting the adjustment for the rotational stall suppression more efficiently while minimizing the pump efficiency drop inevitably caused in the effort of the rotational stall suppression.